At Cray, we believe the questions visionaries pose are a force for discovery, especially those that challenge the limits of possibility. For decades, we have partnered with these visionaries, arming them with advanced and creative supercomputing capabilities so they can question, investigate, and, ultimately, shape the future. As we look ahead, we see opportunities at the convergence of data and discovery—where artificial intelligence (AI) and data analytics unite with supercomputing. We believe Cray is uniquely positioned at this intersection to partner with visionaries who aspire to turn insight into application and discovery into disruption.
Total revenue decreased by $59.6 million for the first six months of 2019 compared to the first six months of 2018, from $199.8 million to $140.2 million, due to lower product revenue. Product revenue was $63.8 million lower in the first six months of 2019 compared to the first six months of 2018, primarily driven by fewer high-dollar value product acceptances in the first half of 2019 as compared to the first half of 2018.
Net loss for the first six months of 2019 was $72.0 million compared to net loss of $36.0 million for the same period in 2018. The increase in net loss was primarily driven by lower product revenue, transaction costs of $7.6 million related to the Merger, and an increase in research and development costs of $13.7 million.
Net cash used in operating activities was $71.2 million for the first six months of 2019 compared to $0.2 million for the first six months of 2018. Net cash used in operating activities for the first six months of 2019 was primarily driven by our net loss, adjusted for non-cash expenses, of $56.0 million, and a $16.4 million decrease in customer contract liabilities as the associated revenue was recognized. These amounts were partially offset by a decrease in accounts receivable of $11.4 million due to payments from customers.